Ofwat has written to the water and wastewater companies setting outs its guidance on how it expects each company to design its public sewerage system to deliver best value for consumers over the long term.

Ofwat has suggested that in the light of evidence on climate change which points to increasing rainfall intensity, companies should take this into account in the design of new assets. In the absence of more accurate guidance companies should take account of increasing rainfall intensity in line with Defra's guidance on climate change1 in planning and designing public sewerage systems.

The guidance recommends that each company should assess whether it is beneficial to future proof new assets rather than replace or amend assets early - which may mean precautionary asset design for the future climate or allowing flexibility so that assets can easily be upgraded.It also sets out Ofwat’s views on sustainable drainage solutions and explains how it expects each company to take account of this for PR09, both in terms of investment that companies undertake to address growth and new development, as well as to maintain base services.

The regulator accepts that companies will not be able to eliminate the risk from sewer flooding, especially with the likelihood that climate change will lead to more extreme rainfall events.  However, Ofwat expects that there will be no net increase in the number of properties judged to have a probability of flooding from sewers more frequently than once in 20 years currently on the sewer flooding registers for each company. The guidance stipulates that design standards must be such that no sewer flooding occurs in storms with a return period of 1 in 20 year or less, with Ofwat only expecting design standards less than this where costs are so exorbitant that the investment is clearly not cost beneficial.

With regard to climate change, Ofwat expects each company to have carried out sufficient climate change sensitivity analysis on investment decisions for their business plans to identify those which are sensitive to potential changes from UKCIP08 (UK Climate Impacts Programme).

The guidance recommends that in planning new sewerage assets companies should use UKWIR's methodology for long term / least cost planning for wastewater supply-demand. This sets out that, where climate change is likely to affect a wastewater system during its asset life, it should be taken into account. Ofwat have also suggested that each company should conduct sensitivity analysis to assess how it should take climate change into account in selecting a cost beneficial solution. In this context each company should consider possible changes to the design to cope with the expected changes to climate over the next hundred years to provide a similar standard of protection from sewer flooding to customers in the future as now.

Each company will be required to explain in its draft business plan for PR09 why it has chosen the flood protection standards that it has planned and how it has assessed that they are in line with consumers' priorities. The companies will also have to set out their assumptions on climate change, how this affects solution selection and the associated expenditure implications. Ofwat has stated

“We will need to be convinced that the evidence is sufficient to justify any increase in costs.”

Ofwat wrote to each sewerage company in February requesting information on its approach to drainage design standards for the public sewerage system. Ofwat subsequently commissioned consultants Atkins to review the responses and make recommendations – Atkins’ report  Development of guidance for sewerage undertakers on the implementation of drainage standards is now available from the Ofwat website.